This invention relates to storage of nuclear fuel rods and more particularly to the consolidation and storage of spent nuclear fuel rods.
After a period of operation of a nuclear reactor, the fuel rod assemblies comprising the core of the nuclear reactor must be rearranged with the depleted or spent fuel assemblies being replaced with fresh ones. The spent fuel assemblies are removed from the reactor vessel and generally stored in a pool of water on the reactor site. Since a conventional fuel assembly comprises structures other than fuel rods such as grids and control rod guide tubes, a spent fuel rod assembly occupies more space in the storage pool than would be required for the individual fuel rods. Because the storage pool has a finite volume, it is desirable to store the fuel rods in a closely packed array and with a minimum of support structure to thereby maximize the amount of spent nuclear fuel that can be stored in a given volume of the storage pool. Increasing the fuel rod packing density increases the available storage capacity for the spent fuel rods until the fuel rods are transported off the reactor site for storage or reprocessing.
However, since the spent fuel rods have been irradiated during reactor operation, they are highly radioactive and can be handled only by remote manipulators and while the fuel rods are submerged in a coolant. The radioactive nature of the spent fuel rod assemblies increases the difficulty of not only transporting the spent fuel rod assembly but of also dismantling the fuel rod assembly and storing the spent fuel rods.
In the above-identified co-pending application, there is described a spent fuel consolidation system for remotely dismantling a spent nuclear fuel rod assembly and removing its fuel rods, while the fuel rod assembly remains submerged in a coolant, and for consolidating the spent fuel rods into a compact array for more dense storage in the storage pool. Referring to FIG. 1A, the spent fuel rod consolidation system, as described in the above-identified application, comprises a rotatable platform 20 that is capable of rotating about its vertical axis under the influence of a drive system (not shown) and that is capable of operating while completely submerged in the storage pool. Platform 20 comprises a vertical support 22, a fuel assembly station 24, a consolidation station 26 and a storage can station 28. Fuel assembly station 24, consolidation station 26, and storage can station 28 are attached to support plate 30 which is rotatably attached to vertical support 22. Support plate 30 is arranged such that when it is rotated about vertical support 22, fuel assembly station 24, consolidation station 26 and storage can station 28 may be selectively positioned with respect to gripper mechanism 32 which is slidably mounted on vertical support 22. A nozzle removal mechanism 34 is also arranged near platform 20 for removing a top nozzle (not shown) from fuel assembly 38. In an illustrative embodiment of this invention, the fuel rod assembly 38 may take the form of that assembly manufactured by Babcock and Wilcox comprised of an array of 15.times.15 rods including 208 fuel rods, 16 guide thimble tubes and a centrally located instrumentation tube. Alternatively, the fuel assembly 38 may be one as described in U.S. Pat. No. 3,791,466 issued Feb. 12, 1974 in the name of J. F. Patterson et al.
In general, fuel assembly station 24 provides a station for holding the spent fuel assembly 38 while its top nozzle and spent fuel rods 40 are removed therefrom. The fuel rods 40 are generally cylindrical metallic tubes containing nuclear fuel as is well understood in the art. Consolidation station 26 supports a transition canister 72, which provides a mechanism for rearranging fuel rods 40 into closely packed configurations. Storage can station 28 provides a station for locating a storage can 84 for accepting and holding fuel rods 40 after fuel rods 40 have been consolidated by the transition canister 72.
The nozzle removal mechanism 34 comprises an internal cutter mechanism 44 that is slidably mounted on positioning mechanism 46. Positioning mechanism 46 serves to position internal cutter mechanism 44 over the fuel assembly 38 originally disposed at the fuel assembly station 24. Since the typical fuel assembly 38 comprises a top nozzle (not shown) which is attached to a plurality of control rod guide tubes 52, it is necessary to cut control rod guide tubes 52 so that the upper portion of control rod guide tubes 52 and the top nozzle may be removed to expose the top ends of the spent fuel rods 40. Positioning mechanism 46 then removes the internal cutter mechanism 44 from the top nozzle.
Next, the internal cutter mechanism 44 is moved away from the fuel assembly station 24 and the gripper mechanism 32 is moved downwardly along vertical member 22 and into contact with the exposed fuel rods 40 of fuel assembly 38. Gripper mechanism 32 then grips each fuel rod 40 as previously described. With gripper mechanism 32 gripping each fuel rod 40, gripper mechanism 32 is moved upwardly along vertical support 22. Since the fuel rod assembly 38 is locked to the fuel assembly station 24, the upward pulling of fuel rods 40 by gripper mechanism 32 removes the fuel rods 40 from the remainder of the fuel rod assembly 38. In this manner, the fuel rods 40 can be removed from the remainder of the fuel assembly 38.
With the gripper mechanism 32 in its uppermost position, platform 20 may be rotated which will cause consolidation station 26 to be positioned under the gripper mechanism 32 and the fuel rods 40. Next, the gripper mechanism 32 is lowered along vertical support 22 so that the fuel rods 40 are inserted into the transition canister 72 originally disposed at the consolidation station 26. The transition canister 72 rearranges the fuel rods 40 as fuel rods 40 are lowered into transition canister 72 thereby closely packing fuel rods 40. When the gripper mechanism 32 has reached its lowermost position, the gripper mechanism 32 releases the fuel rods 40 so that the fuel rods 40 are completely contained in the transition canister 72. Next, the gripper mechanism 32 by means of a conventional gripper (not shown), is caused to grip transition canister 72. While holding the transition canister 72, the gripper mechanism 32 is again raised along the vertical support 22 until the transition canister 72 with the fuel rods 40 therein is raised clear of the consolidation station 26. With the transition canister 72 lifted clear of the consolidation station 26, the platform 20 is again rotated until the storage can station 28 is located under the transition canister 72, as shown in FIG. 1A. When the transition canister 72 is over the storage can station 28, the gripper mechanism 32 is lowered thereby positioning the transition canister 72 on the storage can 84 disposed at the top of storage can station 28. With the transition canister 72 positioned on the storage can 84, the bottom end of the transition canister 72 is remotely opened and the fuel rods 40 disposed into the storage can 84 in a densely packed array or bundle. As shown in FIG. 8C, each storage can 84 may be arranged with a divider so that each storage can 84 can hold more than one set of the consolidated fuel rods 40. Once the fuel rods 40 have been deposited in the storage can 84, transition canister 72 may be returned to the consolidation station 26 by lifting the transition canister 72 and rotating the platform 20 in a reverse direction.
The storage can 84 is the permanent storage depository for one or more bundles of nuclear fuel rods 40. The closely packed bundle of fuel rods 40 needs to be carefully transferred from the transition canister 72 to the storage can 84. Accidental or rough handling of the fuel rods 40 may cause damage thereto and possible release of the nuclear materials contained therein. The release of such nuclear materials could possibly contaminate the reactor site and the storage pool of water.